'\" et
.TH MSGRCV "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
msgrcv
\(em XSI message receive operation
.SH SYNOPSIS
.LP
.nf
#include <sys/msg.h>
.P
ssize_t msgrcv(int \fImsqid\fP, void *\fImsgp\fP, size_t \fImsgsz\fP, long \fImsgtyp\fP,
    int \fImsgflg\fP);
.fi
.SH DESCRIPTION
The
\fImsgrcv\fR()
function operates on XSI message queues (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fImsgrcv\fR()
function shall read a message from the queue associated with the message
queue identifier specified by
.IR msqid
and place it in the user-defined buffer pointed to by
.IR msgp .
.P
The application shall ensure that the argument
.IR msgp
points to a user-defined buffer that contains first a field of type
.BR long
specifying the type of the message, and then a data portion that holds
the data bytes of the message. The structure below is an example of
what this user-defined buffer might look like:
.sp
.RS 4
.nf

struct mymsg {
    long    mtype;     /* Message type. */
    char    mtext[1];  /* Message text. */
}
.fi
.P
.RE
.P
The structure member
.IR mtype
is the received message's type as specified by the sending process.
.P
The structure member
.IR mtext
is the text of the message.
.P
The argument
.IR msgsz
specifies the size in bytes of
.IR mtext .
The received message shall be truncated to
.IR msgsz
bytes if it is larger than
.IR msgsz
and (\fImsgflg\fP & MSG_NOERROR) is non-zero.
The truncated part of the message shall be lost and no indication of
the truncation shall be given to the calling process.
.P
If the value of
.IR msgsz
is greater than
{SSIZE_MAX},
the result is implementation-defined.
.P
The argument
.IR msgtyp
specifies the type of message requested as follows:
.IP " *" 4
If
.IR msgtyp
is 0, the first message on the queue shall be received.
.IP " *" 4
If
.IR msgtyp
is greater than 0, the first message of type
.IR msgtyp
shall be received.
.IP " *" 4
If
.IR msgtyp
is less than 0, the first message of the lowest type that is less than
or equal to the absolute value of
.IR msgtyp
shall be received.
.P
The argument
.IR msgflg
specifies the action to be taken if a message of the desired type is
not on the queue. These are as follows:
.IP " *" 4
If (\fImsgflg\fP & IPC_NOWAIT)
is non-zero, the calling thread shall return immediately with a return
value of \-1 and
.IR errno
set to
.BR [ENOMSG] .
.IP " *" 4
If (\fImsgflg\fP & IPC_NOWAIT) is 0, the calling thread shall suspend
execution until one of the following occurs:
.RS 4 
.IP -- 4
A message of the desired type is placed on the queue.
.IP -- 4
The message queue identifier
.IR msqid
is removed from the system; when this occurs,
.IR errno
shall be set to
.BR [EIDRM] 
and \-1 shall be returned.
.IP -- 4
The calling thread receives a signal that is to be caught; in this case
a message is not received and the calling thread resumes execution in
the manner prescribed in
.IR "\fIsigaction\fR\^(\|)".
.RE
.P
Upon successful completion, the following actions are taken with
respect to the data structure associated with
.IR msqid :
.IP " *" 4
.BR msg_qnum
shall be decremented by 1.
.IP " *" 4
.BR msg_lrpid
shall be set to the process ID of the calling process.
.IP " *" 4
.BR msg_rtime
shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.SH "RETURN VALUE"
Upon successful completion,
\fImsgrcv\fR()
shall return a value equal to the number of bytes actually placed
into the buffer
.IR mtext .
Otherwise, no message shall be received,
\fImsgrcv\fR()
shall return \-1, and
.IR errno
shall be set to indicate the error.
.SH ERRORS
The
\fImsgrcv\fR()
function shall fail if:
.TP
.BR E2BIG
The value of
.IR mtext
is greater than
.IR msgsz
and (\fImsgflg\fP & MSG_NOERROR) is 0.
.TP
.BR EACCES
Operation permission is denied to the calling process; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EIDRM
The message queue identifier
.IR msqid
is removed from the system.
.TP
.BR EINTR
The
\fImsgrcv\fR()
function was interrupted by a signal.
.TP
.BR EINVAL
.IR msqid
is not a valid message queue identifier.
.TP
.BR ENOMSG
The queue does not contain a message of the desired type and
(\fImsgflg\fP & IPC_NOWAIT) is non-zero.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Receiving a Message"
.P
The following example receives the first message on the queue (based on
the value of the
.IR msgtyp
argument, 0). The queue is identified by the
.IR msqid
argument (assuming that the value has previously been set). This call
specifies that an error should be reported if no message is available,
but not if the message is too large. The message size is calculated
directly using the
.IR sizeof
operator.
.sp
.RS 4
.nf

#include <sys/msg.h>
\&...
int result;
int msqid;
struct message {
    long type;
    char text[20];
} msg;
long msgtyp = 0;
\&...
result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
         msgtyp, MSG_NOERROR | IPC_NOWAIT);
.fi
.P
.RE
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess communication
(IPC). Application developers who need to use IPC should design their
applications so that modules using the IPC routines described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fImq_close\fR\^(\|)",
.IR "\fImq_getattr\fR\^(\|)",
.IR "\fImq_notify\fR\^(\|)",
.IR "\fImq_open\fR\^(\|)",
.IR "\fImq_receive\fR\^(\|)",
.IR "\fImq_send\fR\^(\|)",
.IR "\fImq_setattr\fR\^(\|)",
.IR "\fImq_unlink\fR\^(\|)",
.IR "\fImsgctl\fR\^(\|)",
.IR "\fImsgget\fR\^(\|)",
.IR "\fImsgsnd\fR\^(\|)",
.IR "\fIsigaction\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 3.226" ", " "Message Queue",
.IR "\fB<sys_msg.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
